The invention concerns a system for determining at least one parameter of at least one member rotating with respect to a fixed structure, a method of determining at least one such parameter and a motor vehicle comprising such a system.
The invention applies typically to the determination of at least one parameter such as the pressure, temperature, deformation or wear of a motor vehicle tyre.
To do this, it is known how to use, for each tyre:                a transponder which is rotationally fixed to the tyre, the said transponder being able to measure the parameter or parameters; and        a device for detecting the parameter or parameters issuing from the transponder, the said device being fixed to the chassis of the vehicle and comprising an antenna able to transmit an excitation signal to the transponder and to receive the measurement or measurements.        
To allow communication between the transponder and the detection device, the antenna on the latter is positioned close to the area of movement of the transponder, typically in the wheel arch.
Thus, by activating the detection device, it is possible to periodically obtain the value of the parameter or parameters measured by the transponder and to make this value available to a system, for example a safety system on the vehicle, which monitors and/or uses it.
One problem which is posed concerns the establishment of a procedure for activating the detection device which makes it possible to obtain satisfactory communication between the said device and the transponder. This is because, the antenna having a given transmission/reception cone, the activation must be effected when the transponder is situated in this cone.
According to a first solution, it has been proposed to activate the detection device continuously, but this presents several drawbacks. First of all, it gives rise to high electrical consumption, partly unnecessary when the transponder is not situated in the transmission/reception cone. In addition, it acts on the transponder at each rotation, which, under certain running conditions, is unnecessary and, in the case of an active transponder, consumes its battery power unnecessarily.
In order to attempt to limit the electrical consumption, it has been proposed, according to a second solution, to activate the detection device at a fixed frequency. However, this solution also presents several drawbacks. First of all, it does not make it possible to ensure that the transponder is in the transmission/reception cone when the detection device is activated. In particular, for a given activation frequency, there exist speeds of rotation of the transponder for which this condition is not fulfilled, which is incompatible with a use of the parameter or parameters in a safety function for the vehicle. In addition, this solution does not guarantee a determination of the parameter or parameters at a given frequency, for example according to the operating conditions of the vehicle.
In order to attempt to resolve the drawbacks of this second solution, it has been proposed to increase the size of the antenna so as to increase the corresponding transmission/reception cone. However, apart from the fact that this possibility can only limit these drawbacks without completely resolving them since a shadow area in the communication is always present, the increase in the size of the antenna causes increased integration constraints in the wheel arch as well as a higher power and therefore electrical consumption.